For many coating applications such as automotive coatings, aerospace coatings, industrial coatings and architectural coatings, dark colors, such as black and dark blue are particularly desirable for aesthetic purposes. However, dark colored coatings have historically been susceptible to absorption of near-infrared radiation because they often rely on the use of pigments, such as carbon black, that absorb near-infrared radiation in addition to visible radiation. Near-infrared radiation, i.e., light energy having a wavelength of from 700 to 2500 nanometers, constitutes about 45% of the solar energy that reaches the earth's surface. Heat is a direct consequence of the absorption of near-infrared radiation. As a result, dark colored coatings have historically been susceptible to substantially increased temperatures, particularly on sunny days, which is often undesirable for many reasons. Thus, solar heat (near-infrared) reflecting coatings have been desired.
Various approaches for achieving dark solar heat reflecting coatings have been investigated. In one approach, a two layer coating system is employed in which an upper layer is colored with pigments that absorb visible radiation but are transparent to near-infrared radiation, such as organic black pigments (perylene blacks are an example) or other organic pigments (such as phthalocyanine blues and greens and carbazole dioxazine violet), and an underlayer, such as a highly reflective white undercoat, that reflects near-infrared radiation, reduces the temperature increase of the coating system. An example of such a coating system is described in United States Patent Application Publication No. 2004/0191540 A1.
To date, this approach has suffered from some drawbacks. For example, the underlayer, which is often light in appearance, has a tendency to “grin through” in the event that the upper layer is damaged. This can be particularly problematic in applications where appearance is critically important but the coatings are exposed to environments in which coating damage is not uncommon.
Second, the formulation of many dark colored upperlayers are such that they rely on an underlayer (typically a primer) to contribute to the achievement of a proper dark color. A white or lightly colored underlayer will not do this.
Third, in some applications, the underlayer must be opaque at a relatively low film thickness. This is because other coating layers, such as electrodeposited corrosion resisting primers, often lie between the underlayer and the substrate. These coating layers are often susceptible to degradation if exposed to ultraviolet radiation. Hiding of such coating layers by the underlayer must be achieved at a relatively low film thickness due to cost considerations, for example.
As a result, it would desirable to provide solar reflective coatings systems that overcome the deficiencies described above while still providing solar reflection performance comparable to solar reflective coating systems that do not overcome these deficiencies. The invention described herein was made in view of the foregoing desire.